Engineering the Optical Response of the Titanium-MIL-125 Metal–Organic Framework through Ligand Functionalization

Abstract

Herein we discuss band gap modification of MIL-125, a TiO₂/1,4-benzenedicarboxylate (<b>bdc</b>) metal–organic framework (MOF). Through a combination of synthesis and computation, we elucidated the electronic structure of MIL-125 with aminated linkers. The band gap decrease observed when the monoaminated <b>bdc-NH</b>_<b>2</b> linker was used arises from donation of the N 2p electrons to the aromatic linking unit, resulting in a red-shifted band above the valence-band edge of MIL-125. We further explored in silico MIL-125 with the diaminated linker <b>bdc-(NH</b>_<b>2</b><b>)</b>_<b>2</b> and other functional groups (−OH, −CH₃, −Cl) as alternative substitutions to control the optical response. The <b>bdc-(NH</b>_<b>2</b><b>)</b>_<b>2</b> linking unit was predicted to lower the band gap of MIL-125 to 1.28 eV, and this was confirmed through the targeted synthesis of the <b>bdc-(NH</b>_<b>2</b><b>)</b>_<b>2</b>-based MIL-125. This study illustrates the possibility of tuning the optical response of MOFs through rational functionalization of the linking unit, and the strength of combined synthetic/computational approaches for targeting functionalized hybrid materials